Viruses are among the simplest biological systems and are highly effective vehicles for the delivery of genetic material into susceptible host cells. Artificial viruses can be used as model systems for providing insights into natural viruses and can be considered a testing ground for developing artificial life. Moreover, they are used in biomedical and biotechnological applications, such as targeted delivery of nucleic acids for gene therapy and as scaffolds in material science. In a natural setting, survival of viruses requires that a significant fraction of the replicated genomes be completely protected by coat proteins. Complete protection of the genome is ensured by a highly cooperative supramolecular process between the coat proteins and the nucleic acids, which is based on reversible, weak and allosteric interactions only. However, incorporating this type of supramolecular cooperativity into artificial viruses remains challenging. Here, we report a rational design for a self-assembling minimal viral coat protein based on simple polypeptide domains. Our coat protein features precise control over the cooperativity of its self-assembly with single DNA molecules to finally form rod-shaped virus-like particles. We confirm the validity of our design principles by showing that the kinetics of self-assembly of our virus-like particles follows a previous model developed for tobacco mosaic virus. We show that our virus-like particles protect DNA against enzymatic degradation and transfect cells with considerable efficiency, making them promising delivery vehicles.

We report on the biosynthesis of 65 kDa A-B-A triblock copolymers consisting of pH-responsive (acidic) silklike blocks and nonresponsive collagenlike blocks, and we show that at pH values where the silklike blocks become uncharged, these polymers form transparent high-modulus gels, that is, 7-15 kPa at 8 g • L -1 , that consist of supramolecular nanotapes with a height of 2.8 nm, a width of ∼14 nm, and an average length of >10 µm. At the concentrations employed, both of these protein triblocks essentially form the same structure, irrespective of block order. The amount of product isolated from the extracellular medium is in the gram per liter range. This high yield makes various applications of this promising class of biocompatible materials possible.

A custom-designed, highly hydrophilic gelatin was produced in Pichia pastoris. Secreted production levels in single-copy transformants were in the range 3-6 g/l of clarified broth and purification to near homogeneity could be accomplished by differential ammonium sulfate precipitation. Despite the fact that gelatins are highly susceptible to proteolysis because of their unfolded structure, the recombinant protein was shown to be fully intact by SDS-PAGE, N-terminal sequencing, gel filtration chromatography and mass spectrometry. Owing to its highly hydrophilic nature, the migration of the synthetic gelatin in SDS-PAGE was severely delayed. Esterification of the carboxylic amino acid side chains resulted in normal migration. The high polarity of the synthetic gelatin also accounts for its negligible surface activity in water at concentrations up to 5% (w/v), as determined by tensiometry. Circular dichroism spectrometry showed that the non-hydroxylated gelatin did not form triple helices at 4 degrees C. The spectrum was even more representative of the random coil conformation than the spectrum of natural non-hydroxylated gelatins.

Recombinant non-hydroxylated gelatins based on mouse type I and rat type III collagen sequences were secreted from the methylotrophic yeast Pichia pastoris, using the Saccharomyces cerevisiae alpha-mating factor prepro signal. Proteolytic degradation could be minimized to a large extent by performing fermentations at pH 3.0 and by adding casamino acids to the medium, even though gelatin is extremely susceptible to proteolysis due to its open, unfolded structure. Proteolytic cleavage at specific mono-arginylic sites, by a putative Kex2-like protease, could be successfully abolished by site-directed mutagenesis of these sites. Production levels as high as 14.8 g/l clarified both were obtained, using multicopy tranformants. To our knowledge, this represents the highest level of heterologous protein secretion reported to date for P. pastoris.

We studied shear-induced fracture and self-healing of well-defined transient polymer networks formed by telechelic polypeptides, with nodes formed by collagen-like triple helices. When these gels are sheared at a rate that is higher than the inverse relaxation time of the nodes, fracture occurs at a critical stress which increases logarithmically with increasing shear rate. When a constant stress is applied, fracture occurs after a delay time that decreases exponentially with increasing stress. These observations indicate that fracture in these systems is due to stress-activated rupture of triple-helical junctions. After rupture, the physical gels heal completely.

Gelatin hydrogels find broad medical application. The current materials, however, are from animal sources, and their molecular structure and thermal properties cannot be controlled. This study describes recombinant gelatin-like polymers with a general design that inherently offers independent tuning of the cross-link density, melting temperature, and biocompatibility of the gel. The polymers contain small blocks with thermoreversible trimerization capacity and defined melting temperature, separated by hydrophilic nontrimerizing blocks defining the distance between the knot-forming domains. As an example, we report the secreted production in yeast at several g/L of two nonhydroxylated approximately 42 kDa triblock copolymers with terminal trimerizing blocks. Because only the end blocks formed cross-links, the molecular architecture of the gels is much more defined than that of traditional gelatins. The novel hydrogels had a approximately 37 degrees C melting temperature, and the dynamic elasticity was independent of the thermal history. The concept allows to produce custom-made precision gels for biomedical applications.

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We describe a straightforward approach to the covalent immobilization of cytophilic proteins by microcontact printing, which can be used to pattern cells on substrates. Cytophilic proteins are printed in micropatterns on reactive self-assembled monolayers by using imine chemistry. An aldehyde-terminated monolayer on glass or on gold was obtained by the reaction between an amino-terminated monolayer and terephthaldialdehyde. The aldehyde monolayer was employed as a substrate for the direct microcontact printing of bioengineered, collagen-like proteins by using an oxidized poly(dimethylsiloxane) (PDMS) stamp. After immobilization of the proteins into adhesive "islands", the remaining areas were blocked with amino-poly(ethylene glycol), which forms a layer that is resistant to cell adhesion. Human malignant carcinoma (HeLa) cells were seeded and incubated onto the patterned substrate. It was found that these cells adhere to and spread selectively on the protein islands, and avoid the poly(ethylene glycol) (PEG) zones. These findings illustrate the importance of microcontact printing as a method for positioning proteins at surfaces and demonstrate the scope of controlled surface chemistry to direct cell adhesion.